1) Most software requires phurchase, ie uSoft office, Paperport, Blueray player, Nero CD/DVD ect.
2) There is some "free" software such as Open office (Duplicates uSoft Office), Some freebee photo editing/Video editing, Some cd/DVD coping software such as imgburn. You can pick an catagory and do a google search for "free" versions.
3) Many "programs" will have increasing functionallity - As cost increases. Such a with uSoft office has Home/studen/office, Professional and enterprise versions. Some programs will have a "Basic" version that is free and then a "Paid for" version that has more functionallity - Here you would have to go to the web site for the individual program and read what is/is not included in the given version.
DO NOT get "free" pirated copies , NOT only is it illigal, but often comes with Virses/malware.
Depending on what type of software you get, will have certain restrictions on a system level and will require administrator privilages (power user/Root).
To gain access i assume how to download. You can do what ever you like on an "Administrator" user (downlaod/edit registry/veiw files which are normaly hidden).
On a guest account you cant really do anything other than access basic software.